The invention relates to the partial dissolution of natural fibers for the purposes of structural and chemical modifications.
Synthetic polymers such as polystyrene are routinely welded using solvents such as dichloromethane. Natural fiber welding is a process by which biopolymer fibers are fused in a manner roughly analogous to traditional plastic welding. Prior to the discovery that ionic liquids (i.e., 1-ethyl-3-methylimidazolium acetate) can dissolve biopolymers (i.e., cellulose and silk) without derivatization, there were no analogous solvents for natural materials. In addition to the utilization of an ionic liquid, control over the presence and amount of molecular additives (i.e., water, methanol, et cetera) is essential to controlling solvent efficacy. The demonstration of process control by careful manipulation of molecular additives is a fundamental feature that sets this invention apart from previous work. (Previous disclosures and patents exclusively invoke the utilization of neat or “pure” ionic liquids and do not recognize the vital importance of molecular additive control.)
There are many examples in both literature and patents of biopolymer solutions that are cast into molds to create a desired shape. In all of these cases, the biopolymer is completely dissolved so that the original structure is fully disrupted. With fiber welding, the fiber interior is intentionally left in its native state. This is potentially advantageous because the final structure retains some of the original material properties and is significant for creating materials from biopolymers such as silk and cellulose.
Traditional methods of cast molding biopolymer solutions are also disadvantaged in that there is a physical limit to how much polymer can be dissolved in solution. Typically, solutions that are 10% by mass biopolymer (90% by mass ionic liquid solvent) are generally quite viscous and difficult to handle, even at elevated temperatures. The fiber welding process allows (dry) fiber bundles to be manipulated into the desired shape before welding commences. The use and handling of native fibers often grants control over the engineering of the final product that is not possible for solution based technologies.